542 U. S. BUREAU OF FISHERIES 
abundant on the outer third of the gill filaments, where, evidently as 
the result of an irritation caused by their presence, there is a rapid 
proliferation of epithelial cells. This causes the free ends of the 
gill filaments to become greatly enlarged and in some cases distinctly 
club shaped. One of the most striking features of the disease is the 
fact that as a result of the rapid growth of the epithelium, the gill 
filaments often become fused, especially near the tips; and in ex- 
treme cases all the filaments of each gill may become united into a 
continuous mass. 
In addition to the marked proliferation of epithelial cells, there is 
a greatly increased secretion of mucus over the gills, in which par- 
ticles of sand and débris become entangled. Consequently, the gills 
present a very characteristic appearance, which furnishes the only 
reliable means of diagnosing the disease. In other respects the fish 
exhibit no characteristic symptoms, and in fact appear virtually 
normal until a very short time before death. 
Fortunately the disease is controlled easily, one or two treatments 
with a solution of copper sulphate being all that is required. The 
fish were placed in a 1:2,000 solution of copper sulphate for one 
minute and then removed at once to running water. Only a few of 
the weakest fish were injured by the treatment, while nearly all the 
bacteria were destroyed. When followed by a second treatment the 
following day, the bacteria entirely disappeared and there was no 
recurrence of the disease. 
An infection of the fins appeared in two instances among fish that 
were being held in hatchery troughs, but the disease did not spread 
widely. The disease first appeared in a trough of steelhead finger- 
lings but was quickly brought under control by a treatment with 
copper sulphate. Later in the season the same disease broke out in a 
trough of small rainbow fingerlings. In all probability it is a bac- 
terial infection, but owing to the fact that several species of bacteria 
were always present on infected fins, it has not yet been possible to 
determine definitely which was the cause of the disease. 
In most instances the infection is first noticeable on the pectoral 
fins, which become thickened and opaque, but later all the fins may 
become involved. A microscopical examination shows that infection 
first occurs on the outer margin of the fin and appears as a whitish 
discoloration. This is due to a thickening of the epithelium, which 
gradually extends toward the base of the fin. Later, the thickened 
region disintegrates and the fin rays become frayed and broken. 
Eventually the fins may be destroyed entirely, although in the 
majority of cases death intervenes before this occurs. The disease 
can be controlled by the copper-sulphate treatment, as used in the 
case of the gill disease. 
“Fin trouble” is quite common in many hatcheries, and while it 1s 
not always due to the same cause, it is believed that in many cases 
it is the same as that which occurred at Holden. 
During the spring of 1926 there were very heavy losses among the 
advanced brook-trout fry at the Holden station. This mortality oc- 
curred shortly after the fish began to feed, and in one lot the loss was 
virtually 100 per cent. This heavy mortality was confined to fish 
hatched from eggs that had been held in brook water or a mixture of 
brook and spring water. Fish hatched from eggs from the same 
